1. Field of Invention
This invention relates to an ultrasonic flowmeter, and more particularly, to such a flowmeter which is of simple arrangement and which can measure a flow rate with high degree of accuracy.
2. Description of Prior Art
Ultrasonic flowmeters generally have a pair of ultrasonic transducers attached to opposite walls of a pipe through which a fluid to be measured flows. The ultrasonic transducers are usually staggered in the direction of flow of the fluid. The time T.sub.1 in which an ultrasonic wave as transmitted by the transducers in a direction with which the flow propagates, and the time T.sub.2 in which an ultrasonic wave as transmitted by the transducers in a direction against the flow propagates, are different from each other since the speeds of ultrasonic waves in the flowing fluid are subjected to apparent variations due to the speed of flow of the fluid.
These times are expressed as follows: EQU T.sub.1 =l/(C+F.sub.V sin .theta.); T.sub.2 =l/(C-F.sub.V sin .theta.)(1)
wherein C is the velocity of sound propagation in the fluid; F.sub.V is the speed of flow of the fluid; D is the inside diameter of the pipe; .theta. is the angle at which the ultrasonic wave is radiated into the pipe; and l is the distance between the ultrasonic transducers and given by l=D/Cos .theta.. The difference between the reciprocals of the ultrasonic propagation times T.sub.1, T.sub.2 is given by: ##EQU1## In equation (2), sound velocity C in the medium to be measured is eliminated. Since inside diameter D of the pipe and incident angle .theta. are constant, speed V of flow of the fluid can be determined without being affected by sound velocity C by finding the difference between the reciprocals of the ultrasonic propagation times T.sub.1, T.sub.2.
With the speed F.sub.V of flow of the fluid being determined, the flow rate Q.sub.F can be determined by the following: ##EQU2## Accordingly, the ultrasonic flowmeter should be designed to determine ultrasonic propagation times T.sub.1,T.sub.2.
Ultrasonic flowmeters are roughly divided into two categories: (1) sing-around type; and (2) phase-locked loop type. The ultrasonic flowmeter of the sing-around type has a pair of ultrasonic transducers attached in obliquely confronting relation to a pipe through which a fluid to be measured flows. Ultrasonic waves are oscillated alternately from one of the ultrasonic transducers to the other and vice versa. The flow rate of the fluid can be derived from the difference between periods which are the reciprocals of ultrasonic transit times, that is the difference between the oscillation frequencies.
The sing-around type flowmeter is disadvantageous in that when bubbles or dirt, which blocks off ultrasonic wave transmission, passes between the ultrasonic transducer, singaround oscillation is stopped and no measurement is possible. The frequency f of sing-around frequency can be expressed by: ##EQU3##
Assuming in the equation (4) that C=1450 m/sec.; F.sub.V =1 cm/sec.; D=50 mm; and .theta.=22.degree., the difference between sing-around oscillation frequencies, that is, the difference .DELTA.f between a sing-around oscillation frequency f.sub.1 at the time an ultrasonic wave is propagated through the fluid in a direction going with the flow of the fluid and a singaround frequency f.sub.2 at the time an ultrasonic wave is propagated through the fluid in a direction going against the flow of the fluid, becomes 0.14 Hz. If the resolution of flow speed measurement is 1 cm/sec, then the measurement time needed is 2/.DELTA.f.perspectiveto.14 sec., or longer, as there are required a measurement time for ultrasonic propagation in a direction going with the fluid flow and a measurement time for ultrasonic propagation in a direction going against fluid flow. Thus, the sing-around type ultrasonic flowmeter is poor in response.
The flowmeter of the phase-locked loop type is described, for example, in FIG. 1. Components used in measurements based on ultrasonic propagation in directions going with and going against the fluid flow are denoted by reference numerals having suffixes a and b, respectively, and are connected in paired circuits.
Ultrasonic waves are simultaneously radiated with signals from oscillators 2a, 2b into a fluid F to be measured, through ultrasonic transducer 1a,1b attached to outer peripheral surfaces of a pipe T through which fluid F flows. The radiated ultrasonic waves are received by the opposite ultrasonic transducers 1b,1a and then led, as received pulses (amplitude-dependent) to amplifiers 3a, 3b, which are arranged to vary their gains by control signals. The received amplitude-dependent pulses fed to amplifiers 3a,3b are then applied to comparators 4a,4b wherein the amplitudes are compared to preset threshold values. The results of the comparisons are fed, as control signals, back to amplifiers 3a,3b. Thus, amplifiers 3a,3b generates output pulses of constant amplitudes to waveform detectors 5a,5b, respectively, irrespective of the magnitudes of the amplitudes of the received pulses.
Simultaneously with the ultrasonic radiation, outputs from voltage-controlled oscillators 7a,7b are counted respectively by counters 8a,8b, which generate pulses when the counts reach manually set preset count values. Time difference voltage converters 6a,6b are supplied with output pulses from waveform detectors 5a,5b and output pulses from the counters 8a,8b for producing voltage outputs that control the oscillation frequencies of voltage controlled oscillators 7a,7b so that the time differences between the output pulses supplied by the counters 8a,8b to time difference votage converters 6a,6b and the output pulses from waveform detectors 5a,5b are eliminated.
The oscillated outputs from voltage controlled oscillators 7a,7b are led to a differential frequency detector 9 which generates a signal having a frequency equal to the difference between the oscillation frequncies of the voltage controlled osciallators 7a,7b to a frequency-to-voltage converter 10. The signal applied to frequency-to-voltage converter 10 is converted into a voltage proportional to the frequency, which is then applied to a sample-and-hold circuit 11.
A comparator 12 compares the outputs (control signals) from comparators 4a,4b with a preset value of a voltage immediately before it is of a value sufficient for the overall apparatus to operate normally or a lower value, and supplies sample-and-hold circuit 11 with a discrimination signal for determining whether the output from frequency-to-voltage converter 10 is normal or not. When ultrasonic reception is normal, the sample-and-hold circuit 11 delivers the output of frequency-to-voltage converter 10, as it is. When the ultrasonic transfer coefficient of the fluid being measured is lowered, the sample-and-hold circuit 11 continues to supply the normal output immediately prior to the reduction of the ultrasonic transfer coefficient.
With the phase-locked loop system, the difference between the oscillation frequencies of two voltage controlled oscillators 7a,7b becomes smaller as the speed of flow of the fluid being measured, is lowered. Where there are a plurality of oscillators having close oscillation frequencies within one apparatus, the power supply circuits or the oscillation frequencies oscillating through electromagnetic coupling are subjected to mutual interference, resulting in a frequency pull-in phenomenon. The flowmeter has a dead zone due to the frequency pull-in phenomenon in the range wherein the speed of flow of the fluid is low. Stated otherwise, the accuracy of flow rate measurement is poor in the range of low flow speeds.
Solving the above problem requires that the interference between the oscillators be removed. However, designing or packaging a circuit to remove the mutual interference would be difficult to achieve in view of the size and cost of the apparatus that would be involved.
The dead zone is likely to vary since the degree of mutual interference tends to vary due to deterioration of the components. Thus, it would be difficult to keep the measurement accuracy constant for a long period of time.
Another problem with the prior art is that the number of parts required is large and cost is increased, and reliability is lowered. Since the circuit arrangement is composed of many analog circuits, it is susceptible to drift. In addition, the flowmeter suffers from errors during transient conditions, such as immediately after power is turned on or when the flow rate is abruptly changed, because the circuit has a slow response time, and especially because the two circuits have two different response times.